Why do we see stars?

The stars in the distant sky always give people an indescribable attraction.

Exploring the stars is man’s way to the universe. Some people think that each star represents a destiny, others say that the stars are small angels tasked with lighting up the night. Today, science has been able to give us a more precise answer. What is a star? Stars are all celestial bodies that are capable of emitting their own light. All of them are giant air spheres. They are tens to hundreds of thousands of times more massive than Earth. Only thanks to such a large mass can they create their own light. An object to be able to emit its own light needs to have a mass of at least 70 times the mass of Jupiter – the largest planet in the Solar System, that is, about 7% of the mass of the Sun. Why do we see the stars? The stars in the sky have always been a mystery to the human imagination. Our Earth has a mass of about 6x1024kg (6 million billion billion tons). The Sun is 330,000 times heavier than the Earth. That is, a star with a mass of 7% of the mass of the Sun would be about 23,000 times heavier than the Earth. Every object has a gravitational force that directs the center of it to its heart. Normally no one notices but we ourselves are always attracted to our own. Because each part of the body is attracted to each other and the sum of them all form a gravitational force directed towards a center of mass in our body (the center of gravity of the object). The table, the chair, the Earth, are always gravitating to itself by a force called centripetal gravity. But why doesn’t it all burn brightly? That’s because the mass of the objects we come into contact with every day just can’t afford that. Because gravity is a force proportional to mass, gravity in everyday objects is so small that they don’t cause any significant effects. With very large objects such as planets, Earth, gravity is also negligible because it creates a clear attraction that pulls everything towards it. For example, when you jump high, you will fall very quickly because of the pull from the Earth. As for the aforementioned massive objects (tens of thousands of times heavier than the Earth), the great gravity makes the pressure at the center of the celestial body very high, this pressure provides a great acceleration for the celestial bodies. gaseous atoms (mostly hydrogen). They collide strongly with each other at high velocities, breaking the electron shells, separating electrons from the atomic nucleus. At the core of the star is no longer ordinary gas but a state of chaotically moving nuclei and electrons. This state is called plasma. In the plasma state, the hydrogen nuclei have a chance to collide directly with each other at high velocities, which causes what we call fusion reactions, fusing hydrogen nuclei into heavy hydrogen and finally is the helium nucleus. This reaction is known on Earth in hydrogen bombs – bombs capable of releasing thousands of times more energy than atomic bombs of the same mass. The fusion reaction at the core of a star releases a lot of energy in the form of radiation, some of which is visible light. This radiation is transferred to the star’s surface and causes the star to glow. Stars are composed mainly of hydrogen (over 70%), with a large part helium remaining, and an insignificant fraction of heavier gases. The surface temperature of a star is usually in the range of 3,000 to 50,000K, and the temperature at the center is in the range of several million to several tens of millions of K. It can be as high as 100 million K for red giants and several billion K. with red supergiant stars. Star classification Graphic image. By mass, stars are divided into two types, dwarfs and giants. Today, modern division is based on spectral charts. In which, the star with the obtained spectrum of which position on the chart will be determined to belong to which group with specific characteristics of mass and temperature. The most widely used spectrogram today is the Hertzsprung-Russell chart. This graph represents the luminosity, size, and temperature of any star when its spectrum is obtained. According to temperature, the chart is divided into 7 levels with the symbols O, B, A, F, G, K, M respectively. In which, the star closer to O is hotter and closer to M is cold. Each level itself is divided into several sub-levels. Through the chart, it can be seen that most of the stars in the universe are concentrated in the main sequence of the chart. This sequence is a sequence of dwarfs and subgiant stars. Our sun is also on this sequence. It is located in the G group, has the detailed spectral designation G2V (yellow dwarf/Yellow dwarf). Below the sequence are groups of white dwarfs, and above are giants and supergiant, supergiant stars. Star evolution All stars form from large clouds of dust and gas called protostar nebulae. Due to gravity they gather together and shrink until they form a dense mass. As we all know, all objects that carry mass carry gravity. The same object itself also has a force of attraction between different parts of it. However, the gravitational force between small masses is negligible and we hardly notice it. Only significant forces, such as Earth’s gravity acting on people and objects, are enough to be noticed. In stars, gravity is very strong (due to its high mass). When the force of gravity is too great for the atoms to bear, they break the atomic shells and accelerate their nuclei. Hydrogen nuclei (consisting of 1 proton) when collided at high velocity, combine to form heavy hydrogen, and then helium. This reaction releases energy that causes the star to burn brightly. This is a fusion reaction (also known as a nuclear explosion. This reaction is used in the hydrogen bomb (H bomb) – the most destructive destructive weapon that mankind has built). Thanks to the great energy released from nuclear fusion in the star’s core, the gravitational contraction is halted as the released energy balances the gravitational force. The star burns so brightly for several tens, hundreds of millions or billions of years. The lower the mass of the stars, the longer the lifespan. For example, our Sun is a dwarf, medium mass, it can live for about 10 billion years. Meanwhile, stars are much larger, sometimes only living a few hundred or even tens of millions of years because the high mass creates greater pressure towards the center. It causes nuclear fusion to happen faster and the star to deplete energy faster. After burning out all of its hydrogen energy, the star no longer produces energy against centripetal gravity. It will once again shrink. At this time, the helium nuclei combine to form nuclei of heavier elements such as carbon, oxygen and heavier elements up to iron. This process releases an energy that inflates the star’s crust while the star’s core continues to contract. This is the red giant stage. For medium-sized stars (with a mass between 0.5 and 10 times the mass of the Sun), the red giant shell, when inflated sufficiently large, will explode and break up to form a planetary nebula. Meanwhile, high-mass stars have massively inflated stellar shells, becoming red supergiant stars. During this stage, the stellar core continues to contract due to gravity, temperature and pressure both increase many times compared to the previous stage, allowing nuclei of heavier elements to be synthesized (from familiar metals). from copper, silver, and gold to radioactive elements). Up to a certain limit, the energy released from the core creates a large explosion that breaks the outer shell. This is a supervova explosion. After the shell is broken, the star’s core remains for both massive stars as well as light stars. For low- and medium-mass stars like the Sun, the core will stop shrinking, becoming a white dwarf, emitting a very faint light. After billions or tens of billions of years, the generation of radiation ends, stars no longer emit light. It’s called a black dwarf, a dark, dead mass of matter. In fact, the process for a white dwarf to become a black dwarf is so long that so far a black dwarf is only a theoretical prediction. No white dwarf in the universe has been around long enough to become a black dwarf. For massive stars whose core remains after the supervova are at least 1.4 times more massive than the Sun, the mass is so great that they continue to shrink. The nuclei react with each other to form heavy nuclei. The contractions are not over yet, they cause the free electrons to be squeezed tightly against the protons, combining to form neutrons. The star becomes a solid mass of matter, composed entirely of neutrons. Therefore, it has extremely high density and extremely fast rotation speed. This object is called a neutron star. Previously, when this object was first observed, astronomers saw that it emitted a very strong amount of electromagnetic pulses, so they called them pulsars. Even more massive stars with a core mass at least 2 or 3 times that of the Sun, have not stopped after reaching the neutron star stage. They squeeze all matter together to an infinitely large density, concentrated at a location called a singularity. This singularity warps the space around it, a region of space that is bent to an infinite (closed) curvature. The boundary of this space is called the event horizon. Because the space is bent inward, anything that goes in can’t get out, not even light. This entire region of space bounded by the event horizon is called a black hole.